A helicopter rotor blade undergoes various angular movements when the helicopter is in flight. In certain flight conditions the rotor blade is caused to flap, that is, move vertically, at a periodic rate in its rotation about the mast. Flapping is encountered when the helicopter is in forward motion and is due to the differing relative air velocities across the rotor blades. The flapping action of the rotor blades compensates for the differing lift resulting from air velocities across the blades. Flapping is also induced by the operator cyclic control which produces periodic pitch in the rotor blades to cause a tilting of the blade-tip plane. The various flapping motions of the rotor blades as well as other flexible members of the rotor system establish a composite tip-path plane relative to the helicopter fuselage. In most flight conditions this plane is not parallel to the plane of the airframe therefore means must be provided for accommodating the particular tip-path plane encountered for the flying conditions.
Heretofore, blade flapping has been essentially accommodated by a flap hinge in the rotor hub, the hinge permitted the individual blade segments to translate in a plane parallel to the plane of the rotor mast. The flapping of the blade about this hinge induces substantial moments into the helicopter drive train and these moments in turn induce vibratory forces into the helicopter fuselage. Although certain flapping moments are necessary for proper helicopter control, excessive moments generate undesirable vibrations which are harmful to passengers and increase fatigue load on helicopter components. A flapping hinge creates even further vibratory difficulties when the hinge is spaced at greater distances from the rotor mast.
When the entire blade flapping motion is oriented about a single flap hinge or flexure member, the center of gravity of the blade is periodically moved toward the mast and away from the mast during each rotation of the blade. This action causes the blade to correspondingly speed up and slow down in order to maintain constant angular momentum which results in lead-lag motion of the rotor blades about the blade bolt. This lead-lag motion introduces oscillatory shear forces into the rotor hub and these forces are transmitted as vibrations to the helicopter fuselage. Shear forces of this nature are reduced when the extent of blade flapping about a flap hinge in the rotor hub is lessened.
Therefore, there exists a need for a helicopter drive train which can accommodate blade flapping motion and the moments generated thereby without encountering the excessive moments and vibrations due to blade flapping about a fixed hinge in the rotor hub.